This invention relates to a predictive code communication system for use on transmitting video signals, such as television signals or the like.
In a conventional predictive code communication system, a predictive encoder of a recursive type is used to carry out differential pulse code modulation (DPCM). Such a predictive encoder, which is characterized by a feedback loop, comprises a subtractor for subtracting predictive signals from input digital video signals to produce error signals and a quantizer for nonuniformly quantizing the error signals to produce quantized error signals as output digital video signals, as is well known in the art.
The DPCM is effective to reduce or compress an amount of information to be transmitted as the output digital video signals. This is because the DPCM efficiently utilizes a visual characteristic. For example, with the DPCM, degradation of a picture quality is rarely visibly perceived even when an error signal at each picture element is represented by only four bits. At any rate, the encoder input digital video signals are compressed into the predictive error signals by the encoder of the recursive type.
However, quantization errors inevitably accompany the quantized predictive error signals as a result of the above-mentioned nonuniform quantization. Inasmuch as the quantization errors are inevitable in the DPCM, information carried by the encoder input digital video signals is not completely preserved when decoder output digital video signals are produced in a decoder as reproductions of the encoder input digital video signal by the use of a decoding rule reversible to an encoding rule. This also applies to locally decoded digital video signals produced from the quantized predictive error signals in the encoder.
It has been a recent trend that video signals are prevalently edited or dubbed repetitively in a digital manner from an original video tape or tapes to another video tape. In such a case, the video signals must be memorized as digital video signals on each video tape. In order to digitalize such digital video signals, a sampling frequency should be more than 10 megahertz to sample the video signals. Taking the necessity of such high sampling frequency into account, it is preferable that the video signals are memorized on each video tape in the form of compressed digital signals, namely, predictive error signals by allowing the video signals to pass through a predictive encoder of a recursive type, as mentioned before.
In addition, on recording the compressed video signals from the original video tape(s) to another video tape, the compressed video signals should be once decoded into non-compressed, namely, original video signals for the purpose of monitoring video signals to be memorized on another video tape. Especially, monitoring operation is indispensable for recording a plurality of compressed video signals from a plurality of video tapes to another video tape. This is because the compressed video signals may be encoded by the use of encoding rules different from one another.
A decoder as described in conjunction with the recursive type of the predictive encoder must, therefore, be located between the above-mentioned encoder and another video tape.
When editing or dubbing is further repeated, a pair of encoder and decoder, which are similar in structure to those described before, should be connected in cascade to the decoder of the type described. Thus, repetitions of editing or dubbing necessitate a plurality of the aforementioned encoders and decoders. Since the quantization errors are inescapable in each of the encoders of the recursive types, picture qualities are gradually degraded with the repetitions of editing or dubbing owing to accumulation of the quantization errors in each of the encoders and the decoders.
Alternatively, another conventional code communication system comprises a predictive encoder having neither a feedback loop nor a quantizer in the encoder. The predictive encoder of the type described may, therefore, be called an encoder of a non-recursive type. Such a predictive encoder has been proposed in a paper contributed by Robert F. Rice et al to IEEE Transactions on Communication Technology, Vol. COM-19, No. 6 (December, 1971). The proposed encoder comprises an encoder predictor for predicting predictive signals in response to encoder input digital video signals in accordance with a preselected predictive function, namely, an encoding rule and a subtractor for subtracting the predictive signals from the encoder input digital video signals to produce predictive error signals representative of differences between the predictive signals and the encoder input digital video signals, respectively. As a rule, the encoder input digital video signals are given in the form of digital signals subjected to uniform quantization. Such predictive error signals are sent to a decoder as encoder output digital video signals without undergoing quantization.
It is mentioned here that the non-recursive type of the predictive encoder has usually been used in combination with a variable length encoder for encoding the predictive error signal into variable length codes, respectively, in order to reduce an amount of the encoder output digital video signals, as described in the above-referenced paper.
The variable length codes are received by the decoder as decoder input signals and can be decoded into decoder output digital video signals, respectively, without quantization errors by the use of a decoding rule reversible to the encoding rule, after decoded by a variable length decoder. The decoder output digital video signals are reproductions of the encoder input digital video signals and preserve information carried by the encoder input digital video signals. With the system comprising the predictive encoder of the non-recursive type, it is, therefore, possible to preserve information during transmission between the encoder and the decoder.
Because of information preservation, picture qualities are not reduced when the non-recursive types of the predictive encoders are used together with the corresponding decoders for the purpose of repeatedly editing or dubbing video signals in a digital manner. Certainly, each predictive encoder of the non-recursive type is profitable or beneficial on encoding still picture signals, such as facsimile signals. However, the conventional system comprising such a predictive encoder of the non-recursive type is not useful to encode the television signals into the compressed digital signals, as described below.
Herein, let the television signals, such as the NTSC television signals, be given as the encoder input digital video signals to be encoded by the use of the non-recursive type of the encoder. In this case, it should be noted here that the television signals are continuously and rapidly varied with time, different from still picture signals, such as facsimile signals.
Inasmuch as a wide variety of variable length codes appear from the variable length encoder during a long time, with code lengths widely and rapidly changed relative to one another when the television signals are converted to the variable length codes, the amount of the variable length codes sent from the variable length encoder becomes uncontrollable.
Moreover, when the variable length codes are transmitted to the decoder, namely, a receiver through a transmission line, they should be supplied to the transmission line at a specific transmission rate predetermined on the transmission line. It is, however, difficult to transmit the variable length codes at the specific transmission rate on the transmission line because of the uncontrollable amount of the variable length codes.